1. Field of the Invention
The present invention relates to a golf ball, and more particularly to an improvement of a practice golf ball repeatedly used for a long period of time.
2. Description of the Related Art
As is widely known, a golf ball normally has a large number of dimples that are each circular when viewed frontally and are arranged densely for a purpose of improving flight performance of the ball.
A sectional shape of the dimples, especially a practice golf ball, will be described with reference to FIGS. 4 and 5.
FIGS. 4 and 5 are sectional views golf balls in which a radially extended line passes through the center of the dimple.
The contours of the dimple shown in FIG. 4 are such that a wall surface 5 of the dimple 2 is formed in accordance with a circular arc 11 that is a part of a circle 10 with a radius of r centered on a position indicated by reference character C. Accordingly, in FIG. 4, the diameter d of the dimple corresponds to the length of a chord that joins two edges or two points 4, 4 of the dimple 2 at each of which the circular arc 11 and an undimpled portion 3 are divided. A depth f of the dimple is equal to a circular-arc depth g that is downwardly extended from the center of the chord to the circular arc 11 (i.e., bottom 6).
A dimple 2 shown as another example in FIG. 5 is of a pan bottom type in which a wall surface 5 steeply extends from an edge 4, while deviating from a circular arc of a circle 10, to a relatively flat bottom 6. The depth f of the dimple 2 in this case is smaller than the circular-arc depth g.
Since a practice golf ball is used repeatedly for a long period of time, its surface gradually wears down, and thus the dimples become shallow. Accordingly, when the ball continues to be subjected to wear, a decrease in the dimple function, such as a steep rise in trajectory and a reduction in flight distance, appears remarkably.
The present invention was made in consideration of this problem, and aims to provide a golf ball that is not easily worn down in spite of repeated use, and whose performance reduction is slight even if its surface is worn to an extent.
The present invention provides a golf ball having a plurality of dimples on a spherical surface thereof, the dimples each having a circular shape in a plan view,
wherein a virtual dimple is supposed to have a circular shape having the same diameter as the circular shape of the dimple in the plan view, and to be a part of a circle having the same sectional area as that of the dimple in the sectional view;
at least 80% of the dimples have depths not less than 104% of the depth of the virtual dimple, and have a non-circular-arc-shape in a sectional view; and
a total volume of the dimple is in a range of from 300 to 550 mm3.
Since the depth of the dimple is deeply formed for sectional area thereof and the total volume is designed to fall within the above-mentioned numerical range, a necessary function of the dimple can be maintained even if the surface of the ball wears somewhat because of repeated shots as a practice ball. Therefore, it is possible to advantageously prevent a reduction in flight distance and the steep rise in trajectory that are caused by a reduction in dimple depth or a reduction in dimple volume.
In the present invention, the above-mentioned depth of the dimple is at least 4% deeper, preferably in a range of from 8 to 30% deeper, than that of the virtual dimple.
In addition, preferably, the dimple is formed by joining a plurality of different shapes with each other, and the joining portion is positioned at a position of 50% to 90% to the dimple depth.
Preferably, the percentage of the area of all dimples in the entire spherical surface of the ball is 73% or less. If the total area of the dimples is designed to be low like this, it is possible to advantageously prevent the destruction of undimpled portions that is liable to occur because of repeated shots.
As for the structure of the golf ball, the whole thereof can be made of a single kind of elastic material such as rubber (called a one-piece ball). If so made, it is preferable for the ball to have such hardness so as to be deformed by 2.2 to 3.8 mm when a load of 981N (100 Kgf) is applied to the ball placed on a flat plate.
The golf ball can also be made of a composite material structure (i.e., multi-piece solid) in which, for example, a rubber-made core, which has such hardness so as to be deformed by 2.8 to 4.0 mm when a load of 981N (100 Kgf) is applied to the core placed on the flat plate, is disposed at the center of the ball, and the core is covered with a resin cover whose Shore D hardness is 43 to 59. In a usually known ball that is made of a hard core and a cover which is low in hardness, the cover material between a hard club face and the hard core often has its surface shaved off early because of repeated shots. However, if the core and the cover are made such that, as shown in the above-mentioned numerical range, the relatively low-hardness cover material is combined with the largely deformable core, the early destruction of the cover can be prevented effectively.